Our experiments investigate the mechanism of action of excitatory amino acids as synaptic transmitters and neuromodulators in the vertebrate CNS, utilizing cell culture and electrophysiological techniques. Much of our work is on the N- methyl-D-aspartate (NMDA) receptor subtype. Permeation and block of NMDA receptor channels by divalent cations has been investigated using whole cell recording. Ba, Ca, Mn and Sr are all permeant, while Co, Mg, and Ni are voltage dependent blockers. Reversal potential measurements with 0.1 to 50 mM extracellular Ca, and 50 to 150 mM extracellular Na, analysed using an extended constant field equation, show Ca to be approximately 10 times more permeant than Na. However binding of permeant divalent cations is suggested by their block of inward Na current, and alternative models are needed to describe permeation. Low concentrations of zinc and cadmium also block responses to NMDA, however their action is similar at +60 and -60 mV, and thus due to an action at a different site from that for Mg. Zinc acts as a noncompetitive NMDA receptor antagonist, and thus does not interfere with the initial binding of agonist. Fluctuation analysis shows a reduction in open time during Zn and Cd antagonism; possible models under consideration include ultra fast channel block and allosteric modulation to substrates of reduced conductance and lifetime. Excitatory synaptic transmission in hippocampus and spinal cord has been studied under voltage clamp. Epsps are produced by two components of synaptic current: a fast inward current of decay time constant 1-5 ms due to activation of kainate/quisqualate receptors, and a slow component of time constant circa 80 ms due to activation of NMDA receptors. The slow component of the epsp is blocked by selective NMDA receptor antagonists, including low concentrations of zinc, is voltage sensitive in the presence of Mg, and has a Ca-dependent reversal potential. Glycine (1 muM) is a potent modulator of the slow epsp. Conditioned medium from hippocampal glial cell cultures also potentiates the slow epsp and responses to NMDA suggesting that release of modulatory substances from glial cells warrants further investigation. Analytical techniques developed to study the synaptic release process include deconvolution analysis, and a novel nonstationary fluctuation analysis of synaptic currents.